The object of the invention is a device for mixing by turbulence gaseous fluids which contain solid particles in suspension.
The invention is applied more particularly to the calcination of mineral matter in installations for the preparation of cement clinker.
In these installations firing is carried out at high temperature of powdery matter (also called "flour") and consisting generally of a mixture of limestone and clay. The installations for preparation by the dry method include a rotary kiln which serves to roast the flour, and which may be replaced by a fluidized-bed kiln. At the outlet from the kiln the fired flour is cooled by air in a cooler. In compensation, before being introduced into the kiln, the matter undergoes preheating in an installation which consists generally of a series of cyclones in which it circulates in contraflow to hot gases consisting of the flue gases leaving the kiln and/or a device for cooling the matter leaving the kiln.
In general, in the methods of firing silicocalcareous materials, which employ a rotary kiln or one with a fluidized bed and cyclone-type exchangers, it has appeared advantageous in recent years if the greater proportion of the raw material had lost its carbon dioxide before entering the rotary kiln.
That is, it had been realized that the liberation of the carbon dioxide from the slope of the matter in the rotary kiln brought about a destabilization of the flow of the powdery product and a rather irregular arrival of this product in the firing zone of the rotary kiln.
That is why it has appeared well-advised to divide up the application of heat, and to supply between the outlet from the kiln and the exchanger the fuel necessary for the endothermic process of driving off the carbon dioxide. The air necessary for this fuel comes generally from the cooler, either by passing through the kiln or through a duct parallel with the kiln.
This supplementary application of heat is carried out in a system of calcination placed in the circuit of the flue gases between the outlet from the kiln and the lowest cyclone of the preheater.
The powdery product is withdrawn at a temperature of 720.degree. to 780.degree. from the preheater, introduced into the heat-application zone, calcined in this region and then introduced with the flue gases into the bottom cyclone of the exchanger in order to be separated from the flue gases and directed towards the kiln.
It appears that in this type of method the calories produced by the fuel introduced between the outlet from the kiln and the exchanger are employed in an optimal manner if they are absorbed by the endothermic reaction of calcination of the flour between the point of introduction of this flour into the combustion zone and the bottom cyclone of the exchanger, which separates the flour and the flue gases. A less favorable thermal exchange leads to hotter flue gases at the outlet from the bottom cyclone and to a start of calcination in the upper stages of the exchanger.
The temperatures are then raised at all of the stages of the exchanger, and in particular at the outlet of the flue gases from the exchanger, which leads to a lower thermal efficiency of the installation. An effective transfer of the calories applied for the calcination of the flour is therefore necessary for efficient operation of the installation.
The problem has received a first response by a prolongation of the travel of the flour and combustion gases in a very long duct before the cyclone, which may describe loops so as to increase the dwell time necessary to the reaction. Experience shows that it is very difficult in this way to reach lengths which would give the necessary dwell time in this type of flow, and one is led to avoid the loss in efficiency which would result from it in the exchanger by the addition of a supplementary exchanger stage.
But it is known that any increase in height of exchanger towers considerably increases the cost.
Now, the long duration of the dwell times necessary in the solution described above results from the weakness of the turbulence which exists in a duct between the hot gases and the flour.
This has given rise to the idea of intensifying turbulence and by this means speeding up the mixing for homogenization of the flour in the gases and of the temperatures in the gas flows and the heat transfers between gas and flour. Hitherto various systems have been proposed which include mixing chambers intended to intensify the turbulence, in particular by the creation of a vortex flow. The mixing chamber may, for example, have a sidewall of conical shape equipped at its base with an inlet volute and at its upper part with an outlet volute connected to the last cyclone of the preheater. A vortex is produced inside the chamber and a second flow of gas injected along the axis of the outlet volute forms a jet which penetrates inside this vortex in order to be mixed there with the first flow, the whole being discharged through the outlet volute.
Solid matter may be introduced into one or another of the gas flows, and a fuel may likewise be injected into the chamber so as to produce a flame which is mixed with the gases and the matter inside the chamber.
Generally the outlet volute is placed at the upper part of the chamber and the gases forming the vortex are introduced at the lower part by an inlet volute, on the axis of which is placed a cone connected to an exhaust spout in order to recover the solid matter which accumulates by gravity at the time of stoppages or slowing down of the installation. There is a risk of this cone behaving with lower inlet volute as a small cyclone which continually and undesirably collects a portion of the powdery product.
It has likewise been proposed to reverse this device, the inlet volute being placed at the upper part of the chamber and the outlet volute at the lower part. In an embodiment of this type, described, for example, in U.S. Pat. No. 3,098,704, the mixing chamber is associated with a lower chamber into which is introduced air and a fuel which burn in it, the hot gases being exhausted through a flue which penetrates inside the mixing chamber along the axis of the outlet volute.
If solid matter is introduced into such a chamber, there is a risk of the largest fragments, which are not carried along by the flow of gas, accumulating on the bottom wall of the outlet volute round the flue from the combustion chamber.
The object of the invention is an improved device which enables excellent mixing to be carried out by turbulence of two gas flows into which is introduced a solid matter, while assuring the recovery and the recycling of the solid particles which might be deposited at the base of the mixing chamber. In addition, thanks to this recycling, the arrangements enable the dwell time of the solid matter to be prolonged inside the mixing chamber and consequently facilitate the employment of solid fuels having a relatively low calorific power.